1. Field of the Invention
This invention relates to an improvement in a safety apparatus for an elevator system, or more in particular to a safety apparatus capable of overcoming an instantaneous power service interruption which is expected to be restored naturally.
2. Description of the Prior Art
When an abnormal condition occurs while an elevator car is running, the condition is detected and the car is stopped by emergency braking, and kept stationary so as to enable the maintenance personnel to check for the abnormal condition until the recovery of the abnormal condition is attained. A typical means for detecting the abnormal condition is a governor switch for detecting the overspeeding of the car. When this switch is actuated by the detection of the overspeed, the detected condition is mechanically held by a mechanical holding means.
The elevator car, however, is required to be braked in various abnormal situations irrelevant to such mechanical holding means or is naturally braked upon occurrence of certain types of abnormality.
Such a problem is an instantaneous power failure or service interruption wherein a power supply voltage instantaneously drops to zero or a low level and then restores to a normal level.
Generally, an elevator system is supplied with electric power from a substation in the building in which the elevator system is installed (hereinafter referred to as the building substation), which electric power in turn is supplied from an external substation. The other various loads in the building are also supplied with power from the building substation in similar fashion. In the event that an accident occurs in the transmission line, the external substation or the building substation, a power failure may occur even while the elevator car is running. In almost all large buildings, however, power is supplied through double feeder systems and therefore a comparatively short, instantaneous service interruption rather than a prolonged power failure is likely to occur. With buildings rising even higher, elevator car speeds are being increased. Thus in the case of a comparatively short service interruption for an elevator running at a high-speed, it sometimes happens that power is restored and thrown in again before the elevator car comes to a complete stop. In this case, the elevator car to which an emergency brake is already applied due to the power failure will then regain normal operation due to the immediate power restoration only to endanger the passengers by a sudden change of speed from a deceleration to an acceleration.
An especially serious problem is encountered when one of the many relays used in the elevator control system is deenergized for a short period of time by the drop of the source voltage, even if power failure does not occur. If that particular relay has an important function in the elevator operation, the normal elevator operation is interrupted for a short length of time thereby resulting in a sudden change of speed from a deceleration to an acceleration.
The undesirable deenergization of the relay is attributable to various causes including a fault in the relay itself or in the control circuit, the instantaneous drop of the elevator source voltage to the level sufficient to deenergize a particular relay or an instantaneous service interruption.
If the deenergized relay has a function to store information on the direction of movement of the elevator car, on the other hand, the elevator car instantaneously loses the command of movement direction upon the deenergization of the relay and may come to receive a command of the same direction or the reverse direction upon the recovery of power. More specifically, assume that a relay storing the information of "up" direction is actuated and the elevator car is running upward around the seventh floor in response to a tenth-floor call while a fifth-floor call is also present. Under this condition, if the relay storing the "up" direction is deenergized by an abnormality attributable to some cause, the fact that a fifth-floor call has been generated may cause actuation of another relay storing the "down" direction upon the recovery of power. The elevator will then try to turn its direction abruptly, and the resulting sudden change in acceleration will deliver a very great stock both to the passengers and the car, leading to a serious accident.
One of the operating requirements of the elevator car is that the door of the car is completely closed. The door of a running elevator car may be opened instantaneously by mischief or other causes. In this case, too, the elevator car may be subjected to emergency braking and subsequently restore normal operation, so that the speed of the car changes suddenly from a deceleration to an acceleration thereby exposing the passengers to a great danger.